1. Field of the Invention
The present invention relates to a method-of forming a passivation film or a protection film and, in particular, to a method of forming a passivation film of SiO.sub.2 (silicic acid anhydride) on a substrate made of a resinous material or a plastic material.
2. Description of Related Art
As a surface protection film (i.e., a passivation film) to be formed on a resinous material Or a plastic material, attention has recently been paid to SiO.sub.2 and its applications are being expanded. In addition, a passivation film of silicic acid anhydride of superior quality is being desired.
As a method of forming a passivation film of SiO.sub.2 on a substrate of a resinous material or a plastic material, there has hitherto been used a vapor deposition process, a sputtering process, or the like. However, this kind of conventional method has a disadvantage in that, when the surface of the substrate of the resinous material or the plastic material has irregularities in the form of projections and recesses, or curved surface, the adhesion of the SiO.sub.2 film to the surface of the substrate or, in other words, the characteristics of adhesion of the SiO.sub.2 film onto the irregularly shaped portions or step coverage is poor.
As a solution to this disadvantage, there has been used a plasma CVD (Chemical Vapor Deposition) process as a method of improving the adhesion of the SiO.sub.2 film to the surface of the substrate or step coverage and of forming an SiO.sub.2 film of superior quality.
In order to form the SiO.sub.2 film on the surface of the substrate by the plasma CVD process, it is necessary to use an oxidizing gas such as O.sub.2, N.sub.2 O or the like. This plasma CVD process has however the following problem. Namely, the substrate that is made of a resinous material or a plastic material is being subjected to ashing by the radicals of the oxidizing gas. As a result, the surface of the substrate is roughened into a rough surface. In case the surface of the substrate has irregularities in the form of projections and recesses or is curved, its shape is remarkably or largely deformed, with the result that the SiO.sub.2 passivation film of uniform thickness in conformity to the shape of the surface of the substrate cannot be formed.
Therefore, it has long been desired to establish a method of forming an SiO.sub.2 passivation film of good quality on a substrate without subjecting the substrate to ashing.